1. FIELD OF THE INVENTION
The present invention generally relates to shutoff valve structures and more particularly an excess flow shutoff valve for use in hazardous fluid flow lines in which the shutoff valve will be activated to prevent discharge of hazardous fluids from the flow line in the event of abnormal conditions such as rupture of the flow line downstream of the valve or excessive temperature rise.
2. DESCRIPTION OF THE PRIOR ART
In many instances, hazardous fluids must be handled under cotrolled conditions since such fluids may be toxic, flammable or otherwise hazardous if discharged in an uncontrolled manner since release of such fluids to the atmosphere may endanger life and property. It has been the practice and in most instances, there is a requirement that the handling of such materials include a protective valve installed in the piping system to provide a means to protecting personnel and property from accidents caused by excessive product loss due to rupture of pipelines, hose, fittings or equipment connected to the piping. Conventional excess flow valves are designed to close at flow rates in excess of their normal rated flows thus requiring that the rupture in the system permit a flow in excess of the rated flow of the valve in order for it to close. Such valves normally involve a valve disc which is held in an open position by a calibrated spring so that when the flow of product creates a pressure drop across the valve disc greater than the present load on the spring, the valve disc moves against a valve seat to a closed position. Such excess flow valves can be obtained in various flow ratings and since they depend on flow for closure, the line downstream of the excess flow valve must be larger enough to not excessively restrict the flow. Usually, excess flow valves are installed which have a rated closing flow approximately fifty percent greater than the anticipated normal flow. Otherwise, normal flow may cause valve chatter or cause the valve to slug close prematurely during normal operation. For example, if a 90 gpm rated valve is employed in a two inch line and a serious leak develops downstream leaking at the rate of less than 90 gpm, the excess flow valve will remain open and permit flow to the atmosphere thus obviously creating a serious hazard. The leakage rate would have to exceed 90 gpm to cause this valve to close and stop flow. Thus, the uncontrolled flow to the atmosphere must be a high volume in order to cause the presently available type excess flow valve to close. Various patents exist relating to excess flow valves with the following U.S. patents being exemplary of the development of the state of the art. U.S. Pat. Nos. 2,103,747; 2,600,309; 2,887,546; 2,862,051; 3,051,805; 3,113,189; 3,126,463; 3,238,323; 3,283,094; 3,325,612; 3,363,073; 3,549,276; 3,749,864; 3,786,211; 3,822,370; 3,851,127.